1. FIELD OF THE INVENTION
The present invention relates to small sailboats, such as for instance fin keels or windsurfs, and concerns a device to adjust the length of the mast or the boom.
2. DESCRIPTION OF THE PRIOR ART
Mainly in the field of windsurfs, extensible masts are known to allow the use sails having different size. Said masts generally consist of a couple of tubes telescopically introduced the one into the other, the one with larger diameter constituting the base of the mast itself and the one with smaller diameter the adjustable extension. Considering that the sail is tightened at the two ends of the mast, the latter is compressively stressed and the two tubes are blocked in the desired position by simply stopping insertion of the extension tube in the base tube. For this purpose the external circumferential surface of the tube having a smaller diameter presents a series of spaced circumferential grooves, each one corresponding to a reciprocal position of the two tubes and one of which houses a blocking element. Said blocking element is generally constituted by a ring of elastic material or by a coupling capable of radial deformation and presenting a couple of opposed circumferential collars, the inner one of which penetrating into one of said grooves and the outer one urging against the upper edge of the base tube.
In the first case, once the elastic ring is housed in one of said grooves, the extension tube is inserted into the base tube until the ring itself, interfering with a bushing integral to the tube having a larger diameter, stops the insertion of the extension tube.
In the second case, the coupling is radially dilated and moved along the extension tube as far as the desired groove is reached; once said coupling is positioned in correspondence with said groove, it is inserted together with the extension tube into the base tube until its outer collar rests on the edge of the latter.
The main disadvantage of said known devices is in that they do not allow for a continuous adjustment of the insertion of one tube into the other, as the reciprocal locking of the two tubes is obtained by means of said spaced circumferential grooves and moreover they require expensive processing of the grooves with weakening of the inner tube.
A further disadvantage of the devices known up to now is that they are not watertight and thus allow water to penetrate into the tube having a larger diameter. Specially when the boat is used in sea, said water infiltrations generate a salinity fouling inside the tube having a larger diameter, which makes the extension tube sliding difficult.